Hitherto, in the seat belt device attached to a seat of a vehicle such as an automobile, an occupant is constrained by a seat belt in an emergency such as a case in which a large deceleration acts on the vehicle at a collision or the like.
As this type of seat belt devices, a three-point seat belt device is widely known in general and often used. In the known general three-point seat belt device, a seat belt withdrawn from a seat-belt retractor and whose distal end is fixed to the car body is guided by a belt guide toward an occupant. A tongue is slidably supported by the guided seat belt. In that case, the seat belt is inserted through an elongated belt insertion hole of the tongue. And when the tongue is engaged with a buckle fixed to the car body, the seat belt is worn by the occupant. In a state in which the seat belt is worn by the occupant, the seat belt between the belt guide and the tongue functions as a shoulder belt worn by the shoulder and the chest of the occupant, and the seat belt between one end fixed to the car body and the tongue also functions as a lap belt worn by the lap of the occupant. In the above-described emergency, an emergency lock mechanism of the seat-belt retractor operates and prevents withdrawal of the seat belt, whereby forward movement of the occupant by inertia is prevented and the occupant is constrained by the seat belt.
Particularly, in recent years, the seat-belt retractor is provided with a pre-tensioner, and in an emergency, this pre-tensioner is operated in the beginning of the emergency, and the seat-belt retractor retracts the seat belt, whereby looseness of the seat belt is rapidly reduced. As a result, forward movement of the occupant is suppressed, and constraint effect for the occupant is improved. Moreover, the seat-belt retractor is provided with an energy absorption (EA) mechanism, and the seat belt is somewhat withdrawn by this energy absorption mechanism when the emergency lock mechanism operates so that impact energy to the seat belt of the occupant is absorbed and relaxed.
In this type of seat belt devices, in general, even if the emergency lock mechanism of the seat-belt retractor is operated in an emergency, the lap belt is somewhat extended by an inertial force of the occupant, and the occupant somewhat moves forward. Thus, it becomes difficult to effectively constrain the occupant by the lap belt. In that case, even if looseness of the belt is eliminated by the pre-tensioner as described above at occurrence of an emergency, after the operation of the pre-tensioner is finished, that is, after the retraction of the seat belt in the beginning of the emergency, the lap belt can similarly extend by the inertial force of the occupant.
Thus, a tongue is proposed which is configured such that friction between the seat belt and a seat-belt sliding face is reduced in a normal operation of the seat belt and the seat belt slides smoothly with respect to the tongue, but in an emergency, the friction between the seat belt and the seat-belt sliding face increases and the seat belt does not extend to the lap belt side (See PTL 1, for example).
With the tongue described in PTL 1, a large number of axial grooves consecutively extending linearly substantially in the longitudinal direction of a belt insertion hole and a large number of circumferential grooves consecutively extending in a direction substantially orthogonal to the longitudinal direction of the belt insertion hole are provided on the seat-belt sliding face of the belt insertion hole of the tongue. In that case, each axial groove and each circumferential groove are provided on the whole region in the longitudinal direction of the seat-belt sliding face of the elongated belt insertion hole. In the normal operation of the seat belt, the seat belt slides substantially only on the seat-belt sliding face so that friction between the seat belt and the seat-belt sliding face is reduced. As a result, in the normal operation of the seat belt, the seat belt slides smoothly with respect to the tongue. In an emergency, since the seat belt is pulled on the both sides of the tongue by the inertial force of the occupant, the tension of the seat belt increases. By means of the increased tension, the seat belt sinks in the axial grooves and the circumferential grooves, and the friction between the seat belt and the seat-belt sliding face increases. As a result, extension of the seat belt to the lap belt side in an emergency is suppressed. As described above, since extension of the seat belt to the lap belt side is suppressed in the emergency, constraint by the lap belt on the occupant is improved.
Also, in PTL 1, a plurality of projected rims consecutively extending substantially linearly in the longitudinal direction of the belt insertion hole are disposed on the belt sliding face projecting from this belt sliding face and at predetermined intervals in the circumferential direction. It is disclosed that friction between the seat belt and the seat-belt sliding face is changed by these projected rims between the normal operation of the seat belt and the emergency. In that case, in the normal operation of the seat belt, since the seat belt slides only on the surface of each projected rim, the friction between the seat belt and the seat-belt sliding face is reduced. Also, in an emergency, each projected rim falls down to the lap belt side due to the increasing tension of the seat belt, and since the seat belt slides also on the surface of the other portions on the belt-sliding face, the friction between the seat belt and the seat-belt sliding face increases.
Moreover, in PTL 1, it is disclosed that a plurality of resin members projecting from the belt sliding face are provided so that the friction between the seat beat and the seat-belt sliding face is changed by these resin members between the normal operation of the seat belt and the emergency. In that case, in the normal operation of the seat belt, the seat belt is set to slide only on the surface of each resin member. As a result, the friction between the seat belt and the seat-belt sliding face is reduced. Also, in an emergency, it is so configured that each resin member is crushed or cut off by the increasing tension of the seat belt, and the seat belt slides also on the surface of the other portions on the belt sliding face. As a result, the friction between the seat belt and the seat-belt sliding face increases.